The objectives of the proposed studies are to develop computerized mathematical models for the compartmental distribution and metabolism of T4, T3, and rT3 in normal man, which can subsequently be utilized to interpret alterations in thyroid hormone kinetics in a variety of conditions such as nonthyroidal illnesses, fasting, and drug administration. In addition, these models will allow interpretation of serum T3 and rT3 levels following T4 loading in euthyroid athyreotic individuals, thereby localizing the sites of production of T3 and rT3 in vivo.